The invention concerns a device for the electrodynamic spraying of solutions, dispersions and mixtures of solids.
It is known that liquids, such as lacquers and dispersions, for example, or solids, such as powder mixtures, can be sprayed electrodynamically onto a suitable support. The material to be sprayed, such as liquid droplets, or small solids, such as dust or staple fibers, are charged in a strong electrical field and transported by the forces of the electrical field. Known applications are paint and lacquer spraying, coating with solids, such as the manufacture of abrasive paper for example, or of electrofilter materials, and the flocking of textile fibers.
The charging of the particles is effected, for example, by means of ionization grids. The transport of the particles to the counterelectrodes is accomplished in a suitable electrical field of various geometry.
Apparatus and methods are known in which the particles are produced mechanically and only deposited electrically. These are termed mechanical-electrical methods. The disadvantage of such application methods is that the loss of material is considerable. The liquid fogs produced, for example, by spray guns, must be blown at a certain velocity into the area of high ion density to receive the electrical charge, and, on account of the inertia of the particles, the result is that not all of them are sufficiently charged and not all of them are then transported in the electrical field to the counterelectrodes.
If the particles to be sprayed are produced electrically rather than mechanically, this method is referred to as electrostatic or electrodynamic spraying. The charging of the media to be transported in the electrical field is accomplished by contact at the spray electrode. By means of suitable apparatus, a sufficiently great electrical charge is produced, which results in the repulsion of like electrical surface charges, which produces the atomization of liquid films, and also produces the dispersion and spraying apart of bundles of fibers and conglomerates of solids. All of these processes for the production of individual particles at an electrode are referred to as spraying. The particles thus produced are then transported in the electrical field along the lines of force to the counterelectrode, and there they are generally deposited in their entirety.
In contrast to mechanical-electrical spraying, therefore, electrostatic spraying involves hardly any losses of material.
On account of the known advantages of electrodynamic spraying processes, numerous apparatus using a variety of spray electrodes have become known. For example, a rotating bell has been proposed (Ransburg, in "Deutsche Farbenzeitschrift," 1957, p. 132), or a rotating disk whose axis of rotation can have any desired inclination to the counterelectrode (Lurgi, "Deutsche Farbenzeitschrift," 1957, p. 133), a ring electrode (Bayer AG, German Pat. No. 20 32 072), and, in simplest form, a spray electrode having a rectilinear edge (AEG, "Deutsche Farbenzeitschrift," 1957, p. 133). On account of the varying geometry of the spray edge, a specific spray pattern is associated with each electrode shape, which consists in a varying quantity-wise distribution of the spray composition on the counter-electrode. For a defined spraying, a number of requirements must generally be fulfilled. Thus, it is necessary for the electrical field to have as simple a symmetry as possible, and this field symmetry should not vary anywhere in the spray zone. The feeding of the material to the spray edges must be uniform and easy to control. The superimposition of mechanical and electrical forces is undesirable. The spraying process must be independent of the solvent that is used, and the drying of solutions on edges and corners of the spray apparatus must not occur.
The above-stated requirements are not fulfilled by the known electrodes to the necessary extent. Rotating electrodes are especially in need of improvement with regard to field symmetry, to the uniform and controllable feeding of the spray edges, and to the undesired superimposition of mechanical and electrical forces, whereas electrodes having stationary edges have a tendency towards the drying out of solutions especially at the ends of the edges, resulting in each case in severe disturbances of the spraying process.